When spring rolls around, there are visible signals in the produce aisle—bright-green snap peas, funky-looking morels—that make it easy to understand the appeal of seasonal eating. But the seafood section is a different story. Most of us can’t spot the difference between an in-season “local” fluke and the ones we can buy year-round. In addition to navigating buzzwords like line-caught and sustainable, we’re forced to grapple with all sorts of questions about what’s tastiest and—just as importantly—what’s worth the price. Does the rule about “R months” really apply to oysters? (Yes, sort of.) Are lobsters really at their best during the summer? (No, not necessarily.)

Demand for seafood is expected to increase by 35% in the next 20 years due to population growth and its reputation as a staple in a healthy diet. We are already seeing the market’s response: It’s rare to find a restaurant that doesn’t offer salmon, tilapia, or shrimp at any given time during the year—and that applies to sushi restaurants tenfold. But is that a good thing, or is season-less demand leading to sub-standard products and the depletion of the world’s seafood supplies?

To find out why seasonality matters under the sea, we chatted up Charlene Santiago, chef de cuisine of the John Dory. Her short answer is simple: “Seasonality is extremely important for seafood.” But it’s a little more complicated than that. Here’s what you need to know about seasonality, sustainability, and how to the two relate to one another.

Seasonality is tied to taste because when sea creatures are spawning, it affects their flavor.

Santiago says: The general rule that you should only eat shellfish in months that end with the letter R—avoiding the months May through August—is true. Shellfish—take oysters, for instance—spawn during the warmer months. Oysters tend to lose their meatiness and have less taste when they’re from warmer waters. On the other hand, spring is soft-shell crab season. They molt and have soft shells for just a couple of weeks. After spawning, fish are usually following their food and migrating. For example, shad swims up the coast to the cooler waters. When in doubt, check your local seafood seasonality chart.

Sustainability is more important than seasonality.

Santiago says: Most people think of wild striped bass as having dense flesh and fluke as a lighter, less fatty fish, but those characteristics are dependent on how the fish live and what they eat. Just because a fish is farmed doesn’t mean it is not sustainable. Commonly farmed fish—like Atlantic salmon, shrimp, and bivalves—tend to get a bad name, but as with caged chicken and beef, the difference in quality comes down to whether the farmer is raising them and feeding them properly.

Instead, make distinctions based on individual practices and where the fish is coming from. Each country has different environmental regulations; for example, Norway is one of the most stringent.

Eating seasonally reduces the threat of extinction.

Santiago says: Fisherman must follow standards—such as monitoring the size of the fish they can catch, or catching different fish each day—depending on the season. It’s important to pay attention to when fish are spawning and their size. The goal is to avoid catching fish prematurely and hindering reproduction, and also to limit the over-fishing of any one type of fish.

The best way to participate in the natural, seasonal cycle of fishery is to let go of the grocery list. Rather than demanding a specific fish at all times, treat buying fish like the produce in your farm share or farmer’s market. Be open to what is available that day.

Regardless of seasonality, always buy fresh.

Santiago says: Most of the time fishermen freeze fish quickly on the boat and store them in a low temperature on board, so there is no limit on the length of the voyage; the fish will not spoil before they get to land. This can lower the quality, as the flesh can break down and lose its flaky and firm texture. I don’t ever recommend buying frozen fish at the supermarket. It is better to pay a little more at a fishmonger for a fresh fish. Also, if you talk to a fishmonger beforehand and tell him what you are looking for, he can often come up with it from another fisherman for you.

And to clear up another buzzword: Day-boat scallops are fresher, larger, and more tender than scallops caught using a different fishing method.

Santiago says: Scallops retrieved by divers tend to have better quality than netting or a machine dredging through the sand. The scallops are being picked by hand and handled properly; they can also be more consistent in size. This process is more costly and time-consuming than other methods of catching fish and therefore will be more expensive.